Vol. 297, Issue 3, 876-887, June 2001

Antiparkinsonian Agent Piribedil Displays Antagonist Properties at Native, Rat, and Cloned, Human ₂-Adrenoceptors: Cellular and Functional Characterization

Mark J. Millan, Didier Cussac, Graeme Milligan, Craig Carr, Valérie Audinot, Alain Gobert, Franoise Lejeune, Jean-Michel Rivet, Mauricette Brocco, Delphine Duqueyroix, Jean-Paul Nicolas, Jean A. Boutin and Adrian Newman-Tancredi

Departments of Psychopharmacology (M.J.M., D.C., A.G., F.L., J.-M.R., M.B., D.D., A.N.-T.) and Molecular and Cellular Pharmacology (V.A., J.-P.N., J.A.B.), Institut de Recherches Servier, Centre de Recherches de Croissy, Paris, France; and Division of Biochemistry and Molecular Biology, Institute of Biomedical and Life Sciences, University of Glasgow, Glasgow, Scotland, United Kingdom (G.M., C.C.)

Compared with cloned, human (h)D₂ receptors (pK_i = 6.9), the antiparkinsonian agent piribedil showed comparable affinity for h_2A- (7.1) and h_2C- (7.2) adrenoceptors (ARs), whereas its affinity for h_2B-ARs was less marked (6.5). At h_2A- and h_2C-ARs, piribedil antagonized induction of [³⁵S]guanosine-5'-O-(3-thio)triphosphate (GTPS) binding by norepinephrine (NE) with pK_b values of 6.5 and 6.9, respectively. Furthermore, Schild analysis of the actions of piribedil at h_2A-ARs indicated competitive antagonism, yielding a pA₂ of 6.5. At a porcine _2A-AR-Gi1-Cys351C (wild-type) fusion protein, piribedil competitively abolished (pA₂ = 6.5) GTPase activity induced by epinephrine. However, at a _2A-AR-Gi1-Cys351I (mutant) fusion protein of amplified sensitivity, although still acting as a competitive antagonist (pA₂ = 6.2) of epinephrine, piribedil itself manifested weak partial agonist properties. Similarly, piribedil weakly induced mitogen-activated protein kinase phosphorylation via wild-type h_2A-ARs, although attenuating its phosphorylation by NE. As demonstrated by functional [³⁵S]GTPS autoradiography in rats, piribedil antagonized activation by NE of ₂-ARs in cortex, amygdala, and septum. Antagonist properties were also expressed in a dose-dependent enhancement of the firing rate of adrenergic neurons in locus ceruleus (0.125-4.0 mg/kg i.v.). Furthermore, piribedil (2.5-4.0 mg/kg s.c.) accelerated hippocampal NE synthesis, elevated dialysis levels of NE in hippocampus and frontal cortex, and blocked hypnotic-sedative properties of the ₂-AR agonist xylazine. Finally, piribedil showed only modest affinity for rat ₁-ARs (5.9) and weakly antagonized NE-induced activation of phospholipase C via h_1A-ARs (pK_b = 5.6). In conclusion, piribedil displays essentially antagonist properties at cloned, human and cerebral, rat ₂-ARs. Blockade of ₂-ARs may, thus, contribute to its clinical antiparkinsonian profile.